The efficiency and safe performance of a nuclear power plant is often reliant upon a solid understanding of the multiphase flow phenomena that occurs in various components and piping networks throughout the heat transport loops. Depending upon the conditions, various flow regimes can be observed and each of these flow regimes would have a different impact upon the heat transfer and pressure drop of the component or piping that the two-phase mixture is traveling in. Thus, many researchers have performed significant studies to predict flow regime. Over the past several years, the maintenance and inspection efforts at various nuclear power plants have shown that ageing phenomena are occurring including corrosion, cracking, erosion phenomena, and deposition of materials. One of the effects of such phenomena is the roughening of the surface which would impact the nature of fluid behaviour, heat transfer behaviour, and mass transfer at the surface. Relationships for surface conditions and pressure drop/heat transfer are already established for single phase flow conditions albeit with significant uncertainty. Yet, the relationships for multiphase flow have not been established and it is assumed that the surface effects are covered through the single phase component. In this work, experimental studies are performed for a vertical annulus geometry. The apparatus consists of glass tubing to allow visualization of the flow regime inside the tubes. The glass tube material is also a reference case for the smooth tube. Experiments are performed in a bubble column mode for superficial gas velocities up to 0.5 m/s. Observations are taken at various developing lengths and varied water inventories. The results are compared to previous work which shows that the smooth tube results match previous measurements. The inner surface of the tube is modified by forming a thin layer of material over the glass surface. The material is initially moldable and allows for the imprinting of different surface shapes and roughnesses. The material cures in a few hours and is resistant to erosion or dissolution in water. Thus, the surface conditions are changed on the inner tube. The experiments indicate that the flow regime transition from bubbly to slug flow and from slug to churn flow occur at lower superficial gas velocities.

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